The present invention relates to a discharge lamp lighting device and, more particularly, to an improved discharge lamp lighting device with a switching circuit operating in every half cycle of an A.C. power voltage, wherein a choke coil is used as a ballast instead of a step-up transformer in the case where the power voltage is substantially equal to the operating voltage of the discharge lamp.
The conventional ballast for a discharge lamp is composed of, for example, an inductor or a transformer which is mainly made of copper and iron materials, resulting disadvantageously in a large power loss and also a bulky and heavy structure. In view of such situation, lighting devices which have a low power loss and which are small in size and light in weight due to the utilization of semiconductor circuitry have recently been commertialized. FIG. 1 shows a basic arrangement of a lighting circuit which drives a discharge lamp in every half cycle of the A.C. power voltage without use of a transformer even in the case where the operating voltage of the lamp approximates to the power voltage. In the figure, the circuit includes an A.C. current source 1 with a frequency of 50 Hz or 60 Hz, a choke coil 2 functioning as a ballast, a discharge lamp 3, a semiconductor switching circuit 4, which continues its operation also during the lit condition of the lamp 3, and a noise suppressing capacitor 5. In operating the lighting circuit, the switching circuit 4 is closed at the beginning of each half cycle of the power voltage so that a short-circuit current flows through the choke coil 2 whereby to charge electromagnetic energy in it. The switching circuit 4 is cut off when the short-circuit current has reached a predetermined value, i.e. the cut-off current I.sub.cut. The cut-off of the circuit 4 causes the choke coil 2 to generate a high pulse voltage which reignites the lamp 3 in each half cycle of the power voltage. Thereafter, the electromagnetic energy stored in the choke coil 2 is applied to the lamp 3 in addition to the voltage of the source 1 so that the discharge of the lamp may last until the end of the half cycle. Upon the completion of the half cycle, the switching circuit 4 is closed again and the above-mentioned operation is repeated in every following half cycle so that the lamp 3 is maintained in its lit state. The foregoing operation allows the stable lit condition of the lamp without using any transformer even in the case where the operating voltage of the lamp 3 is substantially equal to the voltage of the power source 1.
There have been proposed two circuit systems for the switching circuit 4 in the lighting circuit of this type, one being such a circuit system which performs an ON/OFF operation only once at the beginning of each half cycle, as disclosed in U.S. Pat. No. 3,997,814 entitled "Discharge Lamp Lighting Device", and the other being such a circuit system which performs a repetitive relaxation oscillation between the capacitor 5 and the switching circuit 4 so as to establish an equivalent ON-state, as disclosed in U.S. Pat. No. 4,079,292 entitled "Arc Discharge Sustaining Circuit System for a Discharge Lamp". There are problems, however, in these circuit systems. First, the former circuit system has such disadvantages as will be described hereunder. (1) In the case of a 40-watt energy-saving fluorescent lamp which requires a high pulse voltage for starting thereof, a high starting voltage of approximately 1000-1200 volts is required at a low ambient temperature. Therefore, a switching element having a high blocking voltage is needed as a component of the switching circuit 4. (2) In the case where a large noise suppression capacitor 5 can be used, the capacitor 5 is charged to in excess of one thousand volts by a pulse voltage caused by the cut-off operation of the switching circuit 4 and this electric charge produces a pulse current through the lamp 3 after the latter has been reignited. The peak value of this current may extend to ten times the effective current of the lamp 3, resulting in a short operating life due to the blackening of the lamp wall. On the other hand, if the capacitor 5 is eliminated, a larger cut-off current I.sub.cut is required. In order to achieve this operation, therefore, a switching element having a high blocking voltage and a small power loss is necessary as a component of the switching circuit 4. However, there is no proper switching element presently available for this purpose.
Next, the disadvantages of the latter circuit system are as follows: Since the switching circuit 4 repeats its on and off operations while it has to maintain its ON-state, the virtual period of its ON-state is shortened, resulting in an increased period wherein no lamp current flows. This remarkably deteriorates the efficiency of the lamp, particularly of the fluorescent lamp. In addition, the voltage oscillation by reignition causes high level radio frequency noises.